The final moments prior to when a man orgasms. At this point he can no longer stop himself from ejaculating no matter what happens. He will make a face as though he has just smelled vinegar. It is said that if you peer into a man's eyes during his vinegar strokes you get a glimpse of his soul.

Justin and I accidentally walked in on Kyle during his vinegar strokes. Now we know things about him that no other person can.

The facial expression during sex that coincides with the last thrust before coming.
Imagine the contorted 'fuck face' being pulled by the male as hes about to come, and compare to the wincing one would experience when sucking a lemon, or tasting vinegar. Hence, the vinegar stroke.

Ah man, I was banging this chick, and I was up to the vinegar stroke when my phone rang. Ruined the moment.